Check the pins on the large GA21 SMD custom chip on the right side of the board as well - it drives the RAM chips, and you may have a pin or two loose.
https://shootthecore.tech/repair/repair-log-irem-m92-arcade-pcb/
FWIW, the Shmups forum feedback picture that strikers1945guy posted in his original post that mentions me is legit - I bought a $2000 Cave board from him over a year ago, and it arrived safely, promptly and matched all his descriptions. I sincerely hope this latest transaction comes through for...
Check your voltages for 5.0V. If the board isn’t receiving sufficient power from the power supply then it will definitely cause random RAM errors. Those boards tend to draw more power than your average board too in my experience.
Reporting back on using Namcot Famicom game cases for storing Exa Arcadia games. Can’t say I can recommend them for two reasons:
1) The cart is too small to fit the holders inside the case so it’ll just bang around inside.
2) When the case is closed the “Namcot” manual holder will press down...
If you're sure that the main board is faulty then the cause could be many things. Try probing the graphics RAM from underneath the board with a logic probe and see if there's any stuck or floating logic lines. Start with IC33, 34, 35 and 36.
Also check for loose legs on the 315-5362 custom...
The symptoms you describe make me think one of the address lines for the ROM that stores the sound samples isn't functioning. Could be a broken trace or corrupted ROM. The ROM is labeled "22" and is located at position A5 on the board - check it for broken traces.